The Swing tooltips support html syntax via use of the tag. This can be exploited to show images in tooltips.

For example, with the following layout:

src
   org
      mypackage
         ImageInTooltip.java
         tooltip.gif

and source code:

 package org.mypackage;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class ImageInTooltip {

  public static void main(String[] args) {

    JFrame frame = new JFrame();
    JLabel label = new JLabel("Label with image in Tooltip!");
    label.setToolTipText(
      "<html><img src=\"" +
      ImageInTooltip.class.getResource("tooltip.gif") +
      "\"> Tooltip "
      );
    label.setHorizontalAlignment(JLabel.CENTER);
    frame.setContentPane(label);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setBounds(100, 100, 200, 100);
    frame.setVisible(true);

  }
}

give you this:

Source: sandip chitale's blog